The present invention relates to a circuit and a method for transmitting and receiving an image signal in a high definition television, more particularly to a method and a circuit for transmitting and receiving image signals in low and high frequency band by adaptive modulation to reduce transmission channel noises added to the image signals of the low and high frequency bands when an image signal is transmitted after being image band compressed by subband coding.
Adaptive modulation is suggested in order to reduce the channel noises while transmitting the subband coded signal, in MIT method of the advanced television (ATV) method used in the United States high definition television, and is originated in a concept of pseudo noise quantization to remove degradation of contouring artifact due to the quantization noise.
The image signal transmission and reception technique suggested by the MIT method, one of the United States ATV, uses subband coding as the image band compression algorithm, which is the method that the image band is separated into a high frequency band component and a low frequency band component, and noise reduction process in each band is collectively performed at the transmission part.
Since the image signal of the low frequency band in the above described image signal band compression algorithm is a low frequency component signal in space and time, that is, high correlation signal, the noise of the decoder at the reception part is increased in case that a channel noise is induced into the low frequency band image signal. On the other hand, picture quality will be degraded if the channel noise is not reduced in restoring the image signal of the high frequency band component of low amplitude.
The method of the image signal transmission and reception by adaptive modulation for reducing the noise from the transmission channel adaptively, is presented to solve the above described problems.
A block diagram of the image signal transmission and reception shown in FIG. 1 has an encoder 10 of the transmission part and a decoder 20 of the reception part.
The encoder 10 has an adaption conversion part 11, a delay circuit 12, a multiplier 13 and a nonlinear conversion part 14, The encoder 10 produces a low band image signal of a low band image signal input terminal 10a without noise reduction process. The adaption conversion part 11 receives a high band image signal of a high band image signal input terminal 10b, producing an adaption factor fi and an adaption index information idx. The high band image signal of the high band image signal input terminal 10b is delayed by the delay circuit 12. The multiplier 13 multiplies the delayed high band image signal by the adaption factor fi from the adaption conversion part 11. The multiplied high band image signal by the multiplier 13 is nonlinearly transformed by the nonlinear conversion part 22.
The decoder 20 has a noise reducer 21, a nonlinear conversion part 22, an adaption factor generator 23 and a divider 24. The noise reducer 21 low-pass-filters an input of the low band image signal reception input terminal 20a. An input of the high band image signal reception input terminal 20b is clipped below a given level by the nonlinear conversion part 22. The adaption factor generator 23 is connected to the adaption index information reception input terminal 20c and generates the adaption factor fi corresponding to the adaption index information idx. The divider 24 divides the high band image signal of the nonlinear conversion part 22 by the adaption factor fi from the adaption conversion part 23.
FIG. 2 shows the details of the encoder 10. The adaption conversion part 11 has a absolute circuit 11a, a first and a second buffers 11b and 11c, a first inverter 11d, a first latch 11f, a comparator 11h, a second inverter 11e, an OR gate 11i, a second latch 11g and an adaption factor memory 11j. The absolute value circuit 11a receives the high band image signal from the high band image signal input terminal 10b and makes it to be absolute value. The absolute value of the absolute value circuit 11a is buffered by the first and second buffers 11b and 11c. An interval pulse from the clock pulse terminal CP is applied to the second buffer 11c after being inverted by the inverter 11d. The outputs from the first and second buffers 11b and 11c are latched by the first latch 11f. The comparator 11h compares the output from the absolute circuit 11a with the output from the first latch 11f during a given time T1, producing the larger value among them. The output of the comparator 11h is inverted by the second inverter 11e whose output is applied to the first buffer 11b. The OR gate 11i logically add the output of the comparator 11h and the interval pulse, applying its output to the first latch 11f. The second latch 11g latches out the maximum value of the first latch 11f during the given time T1 depending on the interval pulse. The adaption index information idx and the adaption factor fi are outputted from the adaption factor memory 11j according to the level of the maximum value output from the second latch 11g.
The delay circuit 12 has a memory 12a and a delay buffer 12b. The memory 12a receives the high band image signal of the high band image signal input terminal 10b and produces the high band image signal according to a first-in first-out manner depending on a clock pulse generated from the system clock signal input terminal CK. The high band image signal produced from the memory 12a is delayed by the delay buffer 12b while the adaption factor fi is found.
The image signal transmission and reception technique by the conventional adaptive modulation shown in FIG. 1 and FIG. 2 is described in the Korean application patent serial number 8102 filed on 1990 in detail. The method suggested in the "8102", however, divides only the high band image signal except for the low band image signal into special blocks or adaptive area and carries out adaptive modulation to the respective block by using an adaption factor table.
Therefore, the method and the circuit for transmitting and receiving the image signal by adaptive modulation has the following problems.
First, the channel noise of the low band image signal component can be reduced through the noise reducer by using correlation between the picture elements, but degradation of the picture quality in the edge of the picture is more serious problem rather than the noise reduction effect.
Secondly, since the high band image signal component usually has narrow distribution of its level, the image signal transmission in 6-bit pattern is possible, so that it is waste of the quantity of transmission information that the image signal is transmitted in 8-bit pattern.